1. Field of the Invention
The present invention relates to a method for forming a field oxide film adapted to electrically isolate elements formed on a wafer.
2. Description of the Prior Art
Generally, formation of such a field oxide film has progressed toward reduced occupied area for high integration and generation of reduced stress to a semiconductor substrate for minimized damage of the substrate, in order to ensure provision of active regions and improvement in isolation characteristic.
Referring to FIGS. 1A and 1B, there is illustrated a conventional field oxide formation method, wherein bar type spacers are formed.
Over a silicon substrate 1, first, a pad oxide film 2 and a first nitride film 3 are sequentially deposited in accordance with the illustrated method, as shown in FIG. 1A. The first nitride film 3 is then patterned to a predetermined size. Over the entire exposed surface of the resulting structure, a second nitride film 5 is formed using a chemical vapor deposition (CVD) process.
The second nitride film 5 is then subjected to an etching so as to form nitride film spacers 5', as shown in FIG. 1B. Upon the formation of the nitride film spacers 5', the pad oxide film 2 is partially etched. Thereafter, the resulting structure is subjected to formation of a field oxide film.
However, the first nitride film 3 may be reduced in thickness after the formation of the spacers 5' achieved by etching the second nitride film 5. As a result, an interface is present between the first nitride film 3 and each spacer 5'. Due to such an interface, the spacer 5' may be easily spread on the upper surface of the first nitride film 3, so that its downward pressing force is reduced. Consequently, the spacer 5' can not function effectively to prevent oxygen from being laterally diffused and thereby to inhibit a bird's beak. Since the width of the spacer 5' is also reduced, its use purpose can not be accomplished in the formation of the field oxide film.
In FIGS. 2A and 2B, another conventional field oxide film formation method is illustrated, in which an element isolation is obtained by a trench formed in a semiconductor substrate.
Over a semiconductor substrate 21, first, a pad oxide film 22, a first nitride film 23 and an oxide film 24 are sequentially formed, as shown in FIG. 2A. The oxide film 24 is formed using the CVD process. The oxide film 24 and the first nitride film 23 are sequentially etched to have a predetermined size. A second nitride film 25 is then formed over the entire exposed surface of the resulting structure.
Thereafter, the second nitride film 25 is etched to form nitride film spacers 25'. Using the nitride film spacers 25' as a mask, the pad oxide film 22 and the silicon substrate 21 are sequentially etched so as to form a trench 26 having a depth of d. After the formation of the trench 26, a field oxide film for element isolation is finally formed.
In accordance with this method, however, the first nitride film may be etched during the etching of the silicon substrate. This is because the nitride films and the pad oxide film have similar etch selectivities, respectively. It is also difficult to obtain an accurate depth of the trench because a time etching process should be used to adjust the trench depth.